Surface finishing

ABSTRACT

A method of providing a decorative exposed metallic surface with protection from marring, marking and staining, comprising the steps of coating the surface with an aqueous solution which contains ions of magnesium, ammonium, chromate and phosphate and curing said coating. A composite article comprising a layer of metal having a decorative exposed surface and at least one layer of a substantially water insoluble, transparent, cured coating which offers protection from marring, marking and staining. The cured coating being comprised of magnesium cations and anionic polymeric chains of chromium, oxygen and phosphorus atoms.

United States Patent Boyer [451 Apr. 25, 1972 [73] Assignee: AlleghenyLudlum Steel Corporation, Pittsburgh, Pa.

[22] Filed: Jan.2,1970

21 Appl.No.: 437

[52] US. ..148/6.16, 117/127, 148/627 [51] Int. Cl ..C23f 7/10, C23f7/13[58] FleldofSearch ..148/6.16, 6.15 Z, 6.35,6.27; 117/169, 127

[56] References Cited UNITED STATES PATENTS 2,753,282 7/1956 Perry..148/6.16 3,125,471 3/1964 Conner ....148/6.35 X

Burdick et al.. ..l48/6. 16 Collins ..148/6.16 X

3,248,251 4/1966 Allen ..l48/6.16 X

Primary Examiner-Ralph S. Kendall Attorney-Richard A. Speer, Vincent G.Gioia and Howard R.

Berkenstoclodr.

[ 57] ABSTRACT A method of providing a decorative exposed metallicsurface with protection from marring, marking and staining. comprisingthe steps of coating the surface with an aqueous solution which containsions of magnesium, ammonium, chromate and phosphate and curing saidcoating.

A composite article comprising a layer of metal having a decorativeexposed surface and at least one layer of a substantially waterinsoluble, transparent, cured coating which offers protection frommarring, marking and staining. The cured 1 coating being comprised ofmagnesium cations and anionic polymeric chains of chromium, oxygen andphosphorus atoms.

24 Clalms, No Drawings SURFACE FINISHING The present inventionrelates-to a method for protecting a decorative metallic surface frommarring, marking and staining and to a composite article comprising'alayer of metal and acoating which protects'the metal from marring,marking and staining.

Numerous articles of manufacture, such as waterfountains and buildingpanels have decorative metallic surfaces;.e.g.,.

stainless'steel." These surfaces are in needof protection as they aresubjected to severe abuse. They are scratched, smeared by natural skinoils and other contaminants found on hands, weathered by variousatmospheric conditions and stained-by different solvents and chemicals.

' Considerable timeand money has been spent on developing coatings whichwill protect metallic surfaces. I have discovered that cured coatings ofmagnesium cations and anionic polymeric chains of chromium, oxygen andphosphorus atomsprotectmetal surfaces' without detracting fromtheir'appearance.

It is accordingly an object of this invention to provide a method forprotecting a decorative metallic surface from marring, marking andstaining.

It is another object of this invention to provide a compositearticlecomprising a layer of metal and a coating which protects themetal from marring, marking and staining.

The method of this invention comprises the steps of coating the metal tobe protected with an aqueous solution which contains ions of magnesium,ammonium, chromate and'phosphate and curing the coating so as to renderit substantially water insoluble. In this application, the termphosphate ion includes all ions related to phosphoric acid in an aqueoussolution. Thus, the species H PO.,,-H PO,, HPO, andPO and any polymericphosphorous oxyacidanions are collectively referred to asphosphate'ion.- Similarly, CF05, in solution may exist'as CrO Cr O-,,-etc., aswell as in variousprotonated forms, all oflwhich are included in theterm chromate ion.

The ions; i.e., magnesium, ammonium, chromate and phosphate, maybesupplied' in'various ways. Magnesium ion may be supplied as the'oxide,hydroxide, carbonate, bicarbonate, phosphate-or chromate. Ammonium ionmaybe supplied as uncombined ammonia,.aqueous ammonium hydroxide, or as'the phosphate, chromate, carbonate or bicarbonate salts. Phosphate ionis most conveniently supplied'as orthophosphoric acid. it can alsobesupplied as magnesium phosphate, ammonium phosphate or magnesiumammonium phosphate. Chromate ion may be supplied as chromium trioxide orasthe'chromateor dichromat'e salts of magnesium'or ammonium. When theammonium or magnesiumions-are supplied as their phosphateor chromatesalts, the composition ratios of the solution are limited to thecomposition ratios of the particular salts used. Variations of thesecomposition ratios can, however, be produced by adding extra magnesiumor ammonium as one of the other acceptable compounds. The mostconvenient-method for producing the coating solution is to use magnesiumhydroxide, ortho-phosphoric a'cid, concentrated aqueous ammoniumhydroxide and chromium trioxide.

The magnesium, ammonium,.chromate and phosphate ions are proportionatelypresent in the aqueous solutionas 0.01 28 mole percent magnesium ion, 522 mole percent'ammonium ion, 4 26 mole percent chromate ion and 33 67mole percent phosphate ionand preferably as 16 26 mole percent magnesiumion, 7 mole percent ammonium ion, 7 21 mole percent chromate ion and 4565 mole percent phosphate ion. Magnesium is needed in order to renderthe cured coating substantially water insoluble and to decrease curingtimes to reasonable periods. A lower limit of 0.01 mole percentmagnesium ion is imposed as cured coatings formed from solutions withless than 0.01 mole percent are not water insoluble. An upper limit of28 mole percent magnesium'ion is imposed as 28 mole percent is thelimit-of the solubility for magnesium brain the coating solution.Chromate ion assists in giving the coating its transparency and isrequired to render the coating substantially water insoluble aftercuring. Solutions with less than 4 percent chromate ion produce opaquecured coatingsland. solutions with more than 26.mole percent chromateion produce blotchy coatings which require excessivecuring *times'..Ammonium ion also assistsv in giving the coating its transparency.Solutions with less-than S and more than -22.molepercent"ammoniumionproduce opaque cured coatings; Phosphate ion is required to renderthe solution acidic. A minimum phosphate ion'level off33 mole percentisnecessary in. order for the other ions of the solution to adequately.dissolve. An upper limit of 67 mole percent phosphate ion is set, asgood curing and clarity will not be obtained at=higher levels. When thephosphate ion is in excess of 67rmole percent; there is a deficiencyofother ions.

The concentration of magnesium, ammonium; chromate and.phosphate. ionsinthewater shouldbe no-greater than about iii-moles of ionsper ml. ofwater and preferably should be no greater than 0.8 moles of ions per100ml. of water. 'Solutions'with concentrations in excess of L5 have toohigh a solid level and produce a cloudy, bubbly coat. On the other'hand,there does not appear to be a minimum concentration of-ionsinthe water.Any composition which gives a transparentcoating will continue to give atransparent coating as the amount of water is increased. The only effectof dilution is'to decrease the coating thickness. For practical purposesa lower limit of 0.01 moles'of'ions per 100 ml. of water has been set. Apreferred lower limit is 0.25 moles of ions per 100 ml. of water.

A wetting. agent can be added to the coating solution to insure'the bestpossible wetting action at the metal surface. The use of wetting agentsis optional'and by no means necessary. Typical wetting. agentsareWetanol (Glyco Products, Inc., New York, NY.) and Alkanol HCS (E. l. duPont de Nemours & Company, .Wilmington, Del.

Any of the well-knowncoatingmethodscan be. employed. The invention isnot dependent upon the use of any particular coating'method.Illustrative. coating methods are clip coating, flow coating, spraycoating, roll coating and spin coating.

The. coating should be cured so as to render it substantially waterinsoluble. Curing. is atime and temperaturedependent process. Shortertimes are required at higher temperatures and longer'times are requiredat lower temperatures. Curing temperatures; i.e., the temperatures at:the metallic surfaces, rangingfrom 400" to 1,900" F. have beensuccessfully employed at timesrangingfrom two seconds to four hours.

Various colors and tints can begiven'to the coating by varying'thecuring temperature..Thisenables certain metals; e.g.,. stainless steel,to take. on' the appearance of. color without treating;.e.g., heattinting, themetal. Color' can also be varied by adjusting the relativeproportion-of magnesium, ammonium, chromate and phosphate ions which arepresent in the aqueous coating solution and/or by adjusting thethickness of the cured coating. It is also withinthe scope of thisinvention to add pigments, dies and other chemical agents to alter thecolorof the coating.

The article of thisinvention is comprised of a layer of metal having adecorative surface disposed to be visible and at least one layeroiasubstantially water insoluble, transparent, cured coating which offersprotection from marring,.marking and staining-disposed in adheringrelation on the decorative surface. The coating is comprised. ofmagnesium cations and anionic polymeric chains of chromium, oxygen andphosphorous atoms. No limit is placed on the thickness of the coatingalthough each individual layer should not exceed 0.0005 inches. Layersthicker than 0.0005 inches require an excessive amount of coatingsolution and acquire a frosty appearance during curing.

Although the invention is believed to be adaptable to a wide variety ofmetallic surfaces which include stainless steel, brass, aluminum,silver, zinc, copper, plain carbon steel, lead, chromium, nickel, goldplate, black chrome and platinum;- stainless steel is probably the mostimportant decorative metal and, accordingly, the following is directedto stainless steel embodiments. Stainless steel has a strikinglynoticeable tendency to show scratches and marks which cannot be easilyremoved by any known commercial means without destroying its uniformappearance.

- Stainless steel trim members; e.g., automotive trim members, having acontact surface which abuts a metallic body member less noble in theelectromotive series than stainless steel; e.g., a carbon steelautomotive body, and an exposure surface subject to view, constitute aparticular embodiment of this invention. Another particular embodimentare assemblies comprised of stainless steel trim members and metallicbody members.

Stainless steel is known to serve a particularly good purpose in thoseapplications where brightness and corrosion resistance is required andis ideally suited for use as automotive trim. However, since mostautomotive bodies are constructed of carbon steel, the use of stainlesssteel can result in galvanic corrosion. Carbonsteel is anodic tostainless steel and generally corrodes in the vicinity of stainlesssteel in the presence of an electrolyte, such as moisture. Galvaniccorrosion can, however, be precluded if an insulating material is placedbetween the carbon steel and the stainless steel; e.g., on the contactsurface of the stainless steel trim member. The coating described inthis application is particularly appropriate since it will serve as aninsulator and will also provide the exposure surface with protectionfrom marring, marking and staining.

The following examples illustrate several aspects of the invention.

EXAMPLE I A.I.S.I. Type 434 stainless steel samples 0.0185 inches thickwere dip coated in an aqueous solution containing ions of magnesium,ammonium, chromate and phosphate. The ions were proportionately presentin the solution as 22.5 mole percent magnesium ion, 9.5 mole percentammonium ion, 12.5 mole percent chromate ion and 55.5 mole percentphosphate ion. The molar concentration of the specified ions in thesolution was 0.420 moles of ions per I ml. of water. Half of the sampleshad a bright anneal finish and half had a matte finish. All the sampleswere cured for one minute in a furnace at temperatures ranging from 550'F. up to l,200 F. The bright annealed samples had coatings ranging from0.000040 inches to 0.000l68 inches and the matte finished samples hadcoatings ranging from 0.000043 inches to 0.000209 inches. Table I, foundbelow, gives the appearance of the coating. Note how the appearancechanges with the curing temperature.

TABLE I Curing Curing Appearance Appearance Time Tempera- On Type 434 OnType 434 (Mins.) ture (F.) B.A. Finish Matte Finish 1 min. 550 Dark GoldMedium Yellow Gold 600 Medium Gold Brownish Gold 700 Light Yellow GoldTannish Gray 800 Yellow Gold Tint Light Gray,

Tan Tint s50 Tan Tint Light Gray,

Tan Tint 900 Substantially Gray-Green Colorless Tint 950 SubstantiallyGray-Green Colorless Tint I000 Substantially Gray-Green Colorless Tint lI00 Light Cloudy Gray Gray I200 Cloudy Blue Gray Gray-Tan Tint Thebright annealed and the matte finished samples which were cured at 900F. and l,000 F. were subjected to a number of tests. The favorableresults of the tests are reported below.

SOLVENT, ACID AND ALKALI RESISTANCE No deterioration of the coating wasrevealed after the samples were immersed in acetone, methyl acetate,benzene, kerosene, methyl ethyl keytone, tolulene, methyl alcohol andxylene for a 5-day period.

Concentrated and dilute solutions of nitric, hydrochloric, phosphoric,and acetic acids did not damage the coating after 24 hours of exposure.

Concentrated and dilute hydrofluoric acid caused some deterioration ofthe coating. Concentrated hydrofluoric discolored the coating after 4hours. Dilute hydrofluoric caused a stain at the interface of thecoating and metallic surface after 6 hours.

Concentrated and dilute concentrations of sulfuric acid caused someslight pitting of the sample under the coating.

The continuity; e.g., electrical resistance, of the coating was not,however, afiected after 24 hours of testing.

Concentrated and dilute ammonium hydroxide did not affeet the coatingafter 6 hours.

- Sodium hydroxide at concentrations of 61 percent and 10 percent didnot affect the coating after 6 hours. A 25 percent concentration ofsodium hydroxide heated to a temperature of 200 F., removed all thecoating in 5 to 10 minutes.

SOIL AND ABRASION RESISTANCE Fingermarks and various types of soil wereplaced on the coating and easily removed by wiping.

A bright annealed sample was tested with an abrasion tester. Twoone-half inch wide X 2% inch diameter rubberized carborundum wheels,each causing a 1,000 gram drag on the coated surface, traveled over a 3%inch diameter circle while being driven by the samples which wererevolved. Any coating that will withstand cycles of this test isconsidered to be very good. The tested coating withstood 800 cycleswithout having its continuity impaired. This indicates that it is veryhard and should offer high resistance to marking and marring.

ATMOSPHERIC ENVIRONMENT RESISTANCE Coated samples which were placed in ahumidity cabinet, where the humidity was controlled at 92 percent,revealed no efflorescence or any other detrimental effect.

Coated and uncoated samples were placed on cars in the Pittsburgh, Pa.area for evaluation in a "Winter Corrosion Program" galvanic corrosionwas evident on the cars which had flie 'uiicoatedsamples, but was notevident on the cars which had the'coated samples.

A bright annealed sample was tested in a W'eather-Ometer for 2,292 hourswith no apparent damage to the coating. This is considered to be theequivalent of seven years of atmospherno exposure.

FORMABILITY RESISTANCE No adverse results were observable on shearedsamples. Crazing was observable on the convex side of a coated sampleafter bending.

EXAMPLE II TABLE II Solution A Solution B Ions (Mole (Mole 9E) MagnesiumIon 22.8 21.3 Chromate Ion 18.4

Phosphate lon 55.l

Ammonium ion The molar concentration of the ions in solutions A and Bwere respectively 0.91 and 1.05 moles of the specified ions per 100 ml.of water.

The coated strips were divided into a number of samples which were curedat temperatures ranging from 630 to 1,030 F. for a period of 2 minutes.The samples were subsequently measured for color. Color values wereobtained using a Hunter Associates (Mclean, Va.), Laboratory SphereColor and Color Difference Meter, Model D25.

Table Ill, found below, gives the finish of the samples prior tocoating, the coating solution used, the curing temperature, thecalculated coating thickness after curing and the color values.

marring, marking and staining, comprising the steps of: coating saidmetallic surface with an aqueous solution of magnesium, ammonium,chromate and phosphate ions, said aqueous solution having a molarconcentration of said ions of between about 0.0l and 1.5 moles of saidions per 100 ml. of water, said ions being proportionately present insaid aqueous solution as 0.01-28 mole percent magnesium ion, 522 molepercent ammonium ion, 4-26 mole percent chromate ion and 33-67 molepercent phosphate ion; and curing said coating at an elevatedtemperature so as to render it substantially water insoluble.

2. A method according to claim 1 wherein said ions are proportionatelypresent in said aqueous solution as 16 26 mole percent magnesium ion, 7mole percent ammonium ion, 7 21 mole percent chromate ion and 45 65 molepercent phosphate ion.

TABLElll Curing temp.

Finish Coating solution Thickness of coating (micro inches) Color HazeBrushed Uncoated The L term refers to the lightness of color andcorresponds to the lightness scale used in the Munsell Color System. Lowvalues of L are associated with dull surfaces while bright surfaces givenumerically large L values. L indicates the quantity of black or whitecontained in the hue (color). Higher L values are closer to white andlower L values are closer to black. The a term requires a positive ornegative prefix sign to indicate whether red or green hues are present.The positive prefix is for red hues and the negative prefix is for greenhues. The size of the number indicates the strength or amount of theparticular hue present in the total color. The b" term is similar withthe positive prefix indicating yellowness and the negative prefixindicating blueness. The size of the number indicates the strength oramount of yellow or blue present in the total color. The A E value is acalculated color difference that exists between the selected standardand the sample being examined. It is equal to the square root of (A L)(A a) (A b)? Uncoated samples 1 and 11 were used as the selectedstandard. The haze value is a ratio of the intensity of diffused lightto reflected light times 100. Standard rhodium mirrors have a haze valueof l 1.0 while sandblasted surfaces have haze values near l00.

A study of Table III reveals how a wide range of colors can be impartedto the coating by varying either the coating solution, curingtemperature, material finish and/or coating thickness. The tests show Lvalues ranging from 55.4 to 70.0, a values ranging from 1 .5 to +1.6 andb values ranging from 4.5 to 17.7.

lt will be apparent to those skilled in the art that the novelprinciples of the invention disclosed herein in connection with specificexamples thereof, will suggest various other modifications andapplications of the same. It is accordingly desired that in construingthe breadth of the appended claims they shall not be limited to thespecific examples of the invention disclosed herein.

1 claim:

1. A method of providing a decorative exposed metallic'surface with atransparent cured coating which protects against 3. A method accordingto claim 1 wherein said molar concentration of said ions in said aqueoussolution is between about 0.25 and 0.8 moles of said ions per ml. ofwater.

4. A method according to claim 1 for providing a colored coating whichincludes the step of adding pigments or dies to the aqueous coatingsolution.

5. A method according to claim 1 wherein said coating is cured byheating said metallic surface at a temperature of from about 400 F. toabout l,900 F. for a period of from 2 seconds to about 4 hours.

6. A method according to claim 1 adapted to protect a metallic surfaceselected from the group consisting of stainless steel, brass, aluminum,silver, zinc, copper, plain carbon steel, lead, chromium, nickel, gold,platinum, and alloys of said metals.

7. A method according to claim 6 wherein said metallic surface isstainless steel.

8. A composite article comprising a layer of metal having a decorativeexposed surface and at least one layer of a substantially waterinsoluble, cured coating of magnesium cations and anionic polymericchains of chromium, oxygen, and phosphorus atoms, adhered to saidsurface; said coating having been formed from an aqueous solution ofmagnesium, amrnonium, chromate and phosphate ions having a molarconcentration of between 0.01 and 1.5 moles of said ions per 100 ml. ofwater and having said ions proportionately present as 0.01 28 molepercent magnesium ion, 5 22 mole percent ammonium ion, 4 26 mole percentchromate ion and 33 67 mole percent phosphate ion; said coatingproviding said metallic surface with protection against marring, markingand staining.

9. An article according to claim 8 wherein each layer of said coating isless than about 0.0005 inches thick.

10. An article according to claim 8 wherein said metal is selected fromthe group consisting of stainless steel, brass, aluminum, silver, zinc,copper, plain carbon steel, lead, chromium, nickel, gold, platinum andalloys of said metals.

11. An article according to claim 10 wherein said metal is stainlesssteel.

12. An article according to claim 8 wherein said water insoluble curedcoating is transparent and substantially colorless. I

13. An article according to claim 8 wherein said water insoluble curedcoating is transparent and colored.

14. An article according to claim 8 wherein said metal is stainlesssteel and wherein each layer of said coating is less than about 0.0005inches thick.

15. In the method of producing a stainless steel trim member, wherein astainless steel strip is shaped into a trim member having an exposedsurface and at least one contact surface,'the improvement whichcomprises: coating said contact surface with an aqueous solution ofmagnesium, ammonium, chromate and phosphate ions, said aqueous solutionhaving a molar concentration of said ions of between about 0.01 and 1.5moles of said ions per l ml. of water, said ions being proportionatelypresent in said aqueous solution as 0.01 28 mole percent magnesium ion,22 mole percent ammonium ion, 4 26 mole percent chromate ion and 33 67mole percent phosphate ion; and curing said coating at an elevatedtemperature so as to render it substantially water insoluble.

16. A method according to claim including the steps of coating saidexposure surface with said aqueous solution and curing said coating onsaid exposure surface.

17. A method according to claim 15 wherein said coating is cured byheating said metallic surface at a temperature of from about 400 F. toabout 1,900 F. for a period of from about 2 seconds to about 4 hours.

18. A stainless steel trim member having an exposed surface, a contactsurface and at least one layer of a substantially water insoluble curedcoating of magnesium cations and anionic polymeric chains of chromium,oxygen and phosphorus atoms, adhered to said contact surface; said coat-=ing having been formed from an aqueous solution of magnesium, ammonium,chromate and phosphate ions having a molar concentration of between 0.01and 1.5 moles of said ions per 100 ml. of water and having said ionsproportionately present as 0.01 28 mole percent magnesium ion, 5 22 molepercent ammonium ion, 4 26 mole percent chromate ion and 33 67 molepercent phosphate ion.

null

19. A stainless steel trim member according to claim 18 wherein at leastone layer of said coating is adhered to said exposure surface.

- 20. A stainless steel trim member according to claim 19 wherein eachlayer of said coating is less than about 0.0005 inches thick.

21. An assembly comprising a stainless steel trim member in combinationwith a metallic body member less noble in the electromotive series thanstainless steel; said stainless steel trim member having an exposedsurface, a contact surface abutting said metallic body member and atleast one layer of a substantially water insoluble cured coating ofmagnesium cations and anionic polymeric chains of chromium, oxygen andphosphorus atoms, adhered to said contact surface; said coating havingbeen formed from an aqueous solution of magnesiurn, ammonium, chromateand phosphate ions having a molar concentration of between 0.0l and 1.5moles of said ions per 100 ml. water and having said ionsproportionately present as 0.01 28 mole percent magnesium ion, 5 22 molepercent ammonium ion, 4 26 mole percent chromate ion and 33 67 molepercent phosphate ion.

22. An assembly according to claim 21 wherein at least one layer of saidcoating is adhered to said exposure surface.

23. An assembly according to claim 22 wherein each layer of said coatingis less than about 0.0005 inches thick.

24. A method of providing an exposed metallic surface from the groupconsisting of stainless steel, brass, aluminum, silver, zinc, copper,plain carbon steel, lead, chromium, nickel, gold, platinum, and alloysof said metals with protection against marring, marking, staining, andcorrosion, which comprises the steps of: coating said metallic surfacewith an aqueous solution of magnesium, ammonium, chromate and phosphateions, said aqueous solution having a molar concentration of said ions ofbetween about 0.01 and 1.5 moles of said ions per 100 ml. of water, saidions being proportionately present in said aqueous solution as 0.01 28mole percent magnesium ion, 5 22 mole percent ammonium ion, 4 26 molepercent chromate ion and 33 67 mole percent phosphate ion; and curingsaid coating by heating saidmetallic surface at a temperature of fromabout 400 F. to about l,900 F. for a period of from 2 seconds to about 4hours, so as to render it substantially water insoluble.

2. A method according to claim 1 wherein said ions are proportionatelypresent in said aqueous solution as 16 - 26 mole percent magnesium ion,7 - 15 mole percent ammonium ion, 7 - 21 mole percent chromate ion and45 - 65 mole percent phosphate ion.
 3. A method according to claim 1wherein said molar concentration of said ions in said aqueous solutionis between about 0.25 and 0.8 moles of said ions per 100 ml. of water.4. A method according to claim 1 for providing a colored coating whichincludes the step of adding pigments or dies to the aqueous coatingsolution.
 5. A method according to claim 1 wherein said coating is curedby heating said metallic surface at a temperature of from about 400* F.to about 1,900* F. for a period of from 2 seconds to about 4 hours.
 6. Amethod according to claim 1 adapted to protect a metallic surfaceselected from the group consisting of stainless steel, brass, aluminum,silver, zinc, copper, plain carbon steel, lead, chromium, nickel, gold,platinum, and alloys of said metals.
 7. A method according to claim 6wherein said metallic surface is stainless steel.
 8. A composite articlecomprising a layer of metal having a decorative exposed surface and atleast one layer of a substantially water insoluble, cured coating ofmagnesium cations and anionic polymeric chains of chromium, oxygen, andphosphorus atoms, adhered to said surface; said coating having beenformed from an aqueous solution of magnesium, ammonium, chromate andphosphate ions having a molar concentration of between 0.01 and 1.5moles of said ions per 100 ml. of water and having said ionsproportionately present as 0.01 - 28 mole percent magnesium ion, 5 - 22mole percent ammonium ion, 4 - 26 mole percent chromate ion and 33 - 67mole percent phosphate ion; said coating providing said metallic surfacewith protection against marring, marking and staining.
 9. An articleaccording to claim 8 wherein each layer of said coating is less thanabout 0.0005 inches thick.
 10. An article according to claim 8 whereinsaid metal is selected from the group consisting of stainless steel,brass, aluminum, silver, zinc, copper, plain carbon steel, lead,chromium, nickel, gold, platinum and alloys of said metals.
 11. Anarticle according to claim 10 wherein said metal is stainless steel. 12.An article according tO claim 8 wherein said water insoluble curedcoating is transparent and substantially colorless.
 13. An articleaccording to claim 8 wherein said water insoluble cured coating istransparent and colored.
 14. An article according to claim 8 whereinsaid metal is stainless steel and wherein each layer of said coating isless than about 0.0005 inches thick.
 15. In the method of producing astainless steel trim member, wherein a stainless steel strip is shapedinto a trim member having an exposed surface and at least one contactsurface, the improvement which comprises: coating said contact surfacewith an aqueous solution of magnesium, ammonium, chromate and phosphateions, said aqueous solution having a molar concentration of said ions ofbetween about 0.01 and 1.5 moles of said ions per 100 ml. of water, saidions being proportionately present in said aqueous solution as 0.01 - 28mole percent magnesium ion, 5 - 22 mole percent ammonium ion, 4 - 26mole percent chromate ion and 33 -67 mole percent phosphate ion; andcuring said coating at an elevated temperature so as to render itsubstantially water insoluble.
 16. A method according to claim 15including the steps of coating said exposure surface with said aqueoussolution and curing said coating on said exposure surface.
 17. A methodaccording to claim 15 wherein said coating is cured by heating saidmetallic surface at a temperature of from about 400* F. to about 1,900*F. for a period of from about 2 seconds to about 4 hours.
 18. Astainless steel trim member having an exposed surface, a contact surfaceand at least one layer of a substantially water insoluble cured coatingof magnesium cations and anionic polymeric chains of chromium, oxygenand phosphorus atoms, adhered to said contact surface; said coatinghaving been formed from an aqueous solution of magnesium, ammonium,chromate and phosphate ions having a molar concentration of between 0.01and 1.5 moles of said ions per 100 ml. of water and having said ionsproportionately present as 0.01 - 28 mole percent magnesium ion, 5 - 22mole percent ammonium ion, 4 - 26 mole percent chromate ion and 33 - 67mole percent phosphate ion.
 19. A stainless steel trim member accordingto claim 18 wherein at least one layer of said coating is adhered tosaid exposure surface.
 20. A stainless steel trim member according toclaim 19 wherein each layer of said coating is less than about 0.0005inches thick.
 21. An assembly comprising a stainless steel trim memberin combination with a metallic body member less noble in theelectromotive series than stainless steel; said stainless steel trimmember having an exposed surface, a contact surface abutting saidmetallic body member and at least one layer of a substantially waterinsoluble cured coating of magnesium cations and anionic polymericchains of chromium, oxygen and phosphorus atoms, adhered to said contactsurface; said coating having been formed from an aqueous solution ofmagnesium, ammonium, chromate and phosphate ions having a molarconcentration of between 0.01 and 1.5 moles of said ions per 100 ml.water and having said ions proportionately present as 0.01 - 28 molepercent magnesium ion, 5 - 22 mole percent ammonium ion, 4 - 26 molepercent chromate ion and 33 - 67 mole percent phosphate ion.
 22. Anassembly according to claim 21 wherein at least one layer of saidcoating is adhered to said exposure surface.
 23. An assembly accordingto claim 22 wherein each layer of said coating is less than about 0.0005inches thick.
 24. A method of providing an exposed metallic surface fromthe group consisting of stainless steel, brass, aluminum, silver, zinc,copper, plain carbon steel, lead, chromium, nickel, gold, platinum, andalloys of said metals with protection against marring, marking,staining, and corrosion, which comprises the steps of: coating saidmetallic surface with an aqueous solution of magnesium, ammonium,chromate and phosphate ions, said aqueous solution having a molarconcentration of said ions of between about 0.01 and 1.5 moles of saidions per 100 ml. of water, said ions being proportionately present insaid aqueous solution as 0.01 - 28 mole percent magnesium ion, 5 - 22mole percent ammonium ion, 4 - 26 mole percent chromate ion and 33 - 67mole percent phosphate ion; and curing said coating by heating saidmetallic surface at a temperature of from about 400* F. to about 1,900*F. for a period of from 2 seconds to about 4 hours, so as to render itsubstantially water insoluble.